1. Field of the Invention
The invention generally relates to data communication transmitters such as WLAN (Wireless Local Area Network) transmitters and transceivers, and corresponding methods, and in particular to the adjustment of transmission parameters.
2. Description of the Related Art
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmission, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows for realizing the speed increase.
In 802.11b compliant WLAN systems, all data rates specified in the IEEE 802.11 and 802.11b standards can be used to transmit digital data. The data rates of 1 and 2 Mbps were already possible in the 802.11 standard. The CCK related higher data rates that were introduced by the 802.11b standard, where the data rates of 5.5 and 11 Mbps.
Thus, an 802.11b compliant WLAN transmitter may select one of the above-mentioned four data rates for transmitting its signals. While choosing the highest data rate substantially increases the data throughput since choosing a higher data rate allows for transmitting more data in the same time, this mode cannot be used in all circumstances. For instance, if the present channel conditions are deteriorated by noise, signal reflections, interferences or other negative influences, the actual data throughput may be significantly less than what might be expected. This is because the error rate may increase so that signal retransmissions are required.
Another problem in WLAN systems is to choose the right transmission power. If a transmitter selects a low power level, the above signal deterioration by noise etc may still increase since the signal to noise ratio at the receiver side is decreased. However, choosing a high transmission power may not be the best choice since high power transmissions from one subscriber may then influence the signal quality of data transmissions of other subscribers. Moreover, using higher transmission powers increases the power consumption of the device what is particularly disadvantageous where the transmitter is a mobile station.
A technique for reducing the collision probability in WLAN systems and other data communication systems is the RTS/CTS (Request to Send/Clear to Send) mechanism. The exchange of RTS and CTS frames prior to the actual data frame is one way of distributing medium reservation information announcing the impending use of the medium. While this mechanism may significantly reduce the collision probability in high traffic systems it is not necessary in data communication systems where the channel quality is high. Rather, if the RTS/CTS exchange is performed in high quality systems, the data throughput is even reduced since the exchange of RTS and CTS frames increases the overall traffic volume where this mechanism is not necessary but nevertheless used.
Thus, there are a number of parameters in data communication systems which to choose may be a difficult task. As mentioned above, such parameters may be the data rate, the transmission power and the RTS/CTS threshold that indicates a frame length limit for controlling when to apply this mechanism. There may be many other parameters which need to be adjusted in data communication systems depending on the current channel situation.
In conventional data communication systems, adaptation techniques are applied that adjust one of these parameters to find an optimum and trace this optimum. However, such techniques usually work on only one of these parameters so that multiple mechanisms are required to optimizes all the different aspects mentioned above. This leads to a significant amount of control circuitry necessary to implement such mechanisms, and thus increase the circuit development and manufacturing costs.
Moreover, even when implementing different optimization techniques that each relate to a different one of the above mentioned parameters, there are no synergy effects since the individual optimization techniques would operate completely independently from each other. Moreover, such cumulative optimization mechanisms tend to exhibit instabilities which may occur since controlling one mechanism will somehow influence the conditions that form the basis of controlling another mechanism.